MAX2460 查看數據表（PDF） - Maxim Integrated

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MAX2460

900MHz Image-Reject Transceivers

Maxim Integrated 

900MHz Image-Reject Transceivers

Detailed Description

The following sections describe each of the functional

blocks shown in the Functional Diagram.

Receiver

The MAX2420/MAX2421/MAX2422/MAX2460/MAX2463’s

receive path consists of a 900MHz low-noise amplifier,

an image-reject mixer, and an IF buffer amplifier.

The LNA’s gain and biasing are adjustable through the

LNAGAIN pin. Proper operation of this pin can provide

optimum performance over a wide range of signal lev-

els. The LNA can be placed in four modes by applying

a DC voltage on the LNAGAIN pin. See Table 1, as well

as the relevant Typical Operating Characteristics plots.

At low LNAGAIN voltages, the LNA is shut off, and the

input signal capacitively couples directly into the mixer

to provide maximum linearity for large-signal operation

(receiver close to transmitter). As the LNAGAIN voltage

is raised, the LNA begins to turn on. Between 0.5V and

1V at LNAGAIN, the LNA is partially biased and

behaves like a Class C amplifier. Avoid this operating

mode for applications where linearity is a concern. As

the LNAGAIN voltage reaches 1V, the LNA is fully

biased into Class A mode, and the gain is monotonical-

ly adjustable at LNAGAIN voltages above 1V. See the

Receiver Gain, Receiver IP3, and Receiver Noise

Figure vs. LNAGAIN plots in the Typical Operating

Characteristics for more information.

The downconverter is implemented using an image-

reject mixer consisting of an input buffer with two out-

puts, each of which is fed to a double-balanced mixer.

The local-oscillator (LO) port of each mixer is driven

from a quadrature LO. The LO is generated from an on-

chip oscillator and an external tank circuit. Its signal is

buffered and split into phase shifters, which provide

90° of phase shift across their outputs. This pair of LO

signals is fed to the mixers. The mixers’ outputs are

then passed through a second pair of phase shifters,

which provide a 90° phase shift across their outputs. The

Table 1. LNA Modes

LNAGAIN

VOLTAGE (V)

MODE

0 < V ≤ 0.5

LNA capacitively bypassed, minimum

gain, maximum IP3

0.5 < V < 1.0

LNA partially biased. Avoid this mode—

the LNA operates in a Class C manner

1.0 < V ≤ 1.5 LNA gain is monotonically adjustable

1.5 < V ≤ VCC LNA at maximum gain (remains monotonic)

resulting mixer outputs are then summed together. The

final phase relationship is such that the desired signal is

reinforced and the image signal is canceled. The down-

converter mixer output appears on the RXOUT pin, a sin-

gle-ended 330Ω output.

Transmitter

The transmitter operates similarly to the receiver, but

with the phase shifters at the mixer inputs. The transmit-

ter consists of an input buffer amplifier with more than

36dB of gain-adjustment range via the TXGAIN pin.

This buffer’s output is split internally into an in-phase (I)

and a quadrature-phase (Q) path. IF phase-shifting net-

works give the Q-channel path a 90° phase shift with

respect to the I channel. The I and Q signals are input

to a pair of double-balanced mixers, driven with quad-

rature LO. The mixer outputs are then summed, cancel-

ing the image component. The image-rejected output

signal is fed to the PA predriver, which outputs typically

-3dBm on the TXOUT pin.

Since the transmit and receive sections share an LO

and an IF frequency, interference results if both sec-

tions are active at the same time.

Phase Shifters

MAX2420/MAX2421/MAX2422/MAX2460/MAX2463

devices use passive networks to provide quadrature

phase shifting for the receive IF, transmit IF, and LO

signals. Because these networks are frequency selec-

tive, proper part selection is important. Image rejection

degrades as the IF and RF move away from the

designed optimum frequencies. The MAX2420/

MAX2421/MAX2422’s phase shifters are arranged such

that the LO frequency is higher than the RF carrier fre-

quency (high-side injection), while the MAX2460/

MAX2463’s phase shifters are arranged such that the

LO frequency is lower than the RF carrier frequency

(low-side injection). Refer to the Selector Guide.

Local Oscillator (LO)

The on-chip LO is formed by an emitter-coupled differ-

ential pair. An external LC resonant tank sets the oscil-

lation frequency. A varactor diode is typically used to

create a voltage-controlled oscillator (VCO). See the

Applications Information section for an example VCO

tank circuit.

The LO may be overdriven in applications where an

external signal is available. The external LO signal

should be about 0dBm from 50Ω, and should be AC

coupled into either the TANK or TANK pin. Both TANK

and TANK require pull-up resistors to VCC. See the

Applications Information section for details.

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